Crystal structure and photoluminescence of a new two-dimensional Cd(II) coordination polymer based on 3-(carboxymethoxy)-2-naphthoic acid

2.1 Solid state structure of 1

Selected bond lengths and angles for [Cd(CNA)]_n (1) are listed in Table 1. X-ray crystallographic analysis has shown that crystals of 1 are monoclinic, space group P2₁/c. As is illustrated in Fig. 1, the asymmetric unit contains one unique Cd(II) cation and one unique CNA anion. Each Cd(II) atom is seven-coordinated by seven carboxylate O atoms from three different CNA anions in a distorted pentagonal-bipyramidal configuration. The Cd–O distances range from 2.251(3) to 2.615(2) Å, which are normal and comparable with the counterparts found in the literature [4]. Each CNA anion bridges three Cd(II) cations through its two carboxylate groups and one –O– group. Adjacent Cd(II) cations are thus linked by the CNA anions to give rise to a two-dimensional network structure (Fig. 2). In the network, the Cd···Cd separations by CNA anions are 4.53 and 4.42 Å, respectively. The naphthalene groups of the CNA anions are alternately located on both sides of the network. It is noteworthy that π–π stackings and hydrogen-bonding interactions have not been found between neighboring networks. From the topological point of view, each CNA anion can be regarded as a linker, and each Cd(II) cation can be considered as a 4-conncted node. Thus, the whole two-dimensional network of 1 is a 4-connected (4,4) network. It should be pointed out that although various Cd(II)-containing coordination polymers based on polycarboxylates have been reported thus far, examples constructed by polycarboxylates with both flexible and rigid carboxylate groups have been rarely observed.

Fig. 1:

ORTEP view of 1 showing the local coordination environments of the Cd(II) center with hydrogen atoms omitted for clarity (displacement ellipsoids at the 30 % probability level). Symmetry codes: (A) x, 1/2 − y, −1/2 + z; (B) 2 − x, 1/2 + y, 3/2 − z.

Fig. 2:

View of the two-dimensional network structure of 1 constructed by CNA anions and Cd(II) cations.

2.2 Thermal behavior

In order to study the thermal behavior of the compound, a thermogravimetric analysis (TGA) was carried out. The experiment was conducted under N₂ atmosphere with a heating rate of 10 °C min⁻¹ from room temperature to 800 °C. As shown in Fig. 3, compound 1 is thermally stable up to around 217 °C. The weight loss in several steps corresponds to the decomposition of the CNA anion in the temperature range of 217–535 °C (obsd. 65.8 %, calcd. 68.5 %). The residual weight 34.2 % (calcd. 36.0 %) corresponds to CdO. This result is in good accordance with the formula of the title compound.

Fig. 3:

TG curve of compound 1.

2.3 Photoluminescence properties

The d¹⁰- or p-block metal-based coordination polymers have received increasing interest because of their intense luminescence as well as potential applications in photochemistry, as chemical sensors, and in electroluminescent displays [18, 19]. In this work, the solid state photoluminescent properties of H₂CNA and compound 1 were studied at room temperature. As is shown in Fig. 4, H₂CNA and compound 1 exhibit strong fluorescence at room temperature. The main emission peak of H₂CNA is at 506 nm (λ_ex = 325 nm). This emission band for the free ligand H₂CNA can be assigned to the π* → n or π*→ π transitions [18]. Upon complexation with the Cd(II) cation, the emission peak of 1 appears at 384 nm (λ_ex = 325 nm), highly blue-shifted with respect to the free H₂CNA ligand. The observed blue shift of the emission maximum of 1 may arise from the variation of the highest occupied and lowest unoccupied molecular orbital levels upon deprotonation and metal coordination [4].

Fig. 4:

Emission spectrum of 1 excited at 325 nm.

3.1 General

All materials were of analytical or reagent grade and used as received without further purification (Sinopharm Chemical Reagent Co. Ltd, China). The photoluminescence properties of the ligand and compound were measured on a Renishaw in a Via Raman Microscope (Renishaw Company, UK). TGA were performed on a Perkin-Elmer TG-7 analyzer (Perkin-Elmer Company, USA) from 20 to 800 °C under nitrogen atmosphere with a heating rate of 10 °C min⁻¹. Elemental analysis was carried out with a Perkin-Elmer 240C analyzer (Perkin-Elmer Company, USA).

3.2 Synthesis of compound 1

CdCl₂·2.5H₂O (0.2 mmol, 0.046 g), H₂CNA (0.12 mmol, 0.032 g), and KOH (0.4 mmol, 0.022 g) were dissolved in methanol–isopropyl alcohol–H₂O (1:1:3) (10 mL), and the mixture was stirred for 1 h at room temperature. Then the mixture was transferred and sealed in a 15 mL Teflon-lined stainless steel container. The container was kept at 443 K for 4 days. After the mixture had been cooled to room temperature at a rate of 10 °C h⁻¹, colorless block crystals of 1 were obtained in a yield of 17 % based on Cd(II). – Anal. for C₁₃H₈O₅Cd (%): calcd. C 43.78, H 2.26; found: C 43.14, H 2.13.

3.3 X-ray structure determination

The crystallographic data of 1 were collected on a Bruker-AXS Smart CCD diffractometer with graphite- monochromatized MoK_α radiation (λ = 0.71073 Å) by using an ω–ϕ scan method at 293(2) K. Absorption corrections were applied using multiscan techniques. The structure was solved by Direct Methods (Shelxs-97) and refined by full-matrix least-squares techniques using the program Shelxl-97 [20, 21]. The non-hydrogen atoms of the compounds were refined anisotropically. The hydrogen atoms attached to carbons were generated geometrically.

CCDC 1048752 (1) contains the supplementary crystallographic data for this paper (Table 2). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.